Motor vehicles that have automatic transmissions usually employ torque converters positioned between the flexplate or flywheel of the vehicle's engine, and the input shaft of the transmission. In general, the torque converter offers two main advantages. First, at low engine speeds, torque converters multiply the torque, usually by a ratio of about two to one, produced at the flywheel, thereby providing increased power to the driven wheels of the vehicle. Second, the torque converter allows the car to stop and idle without disconnecting the engine from the rest of the drive train.
Most torque converters consist of three main components; the impeller, the turbine, and the stator. The impeller is generally bolted directly to the flexplate and rotates at engine speeds. The turbine is usually coupled to the input shaft of the vehicle's transmission. The stator is positioned between the impeller and the turbine and is equipped with a one way clutch. All three components have canted blades and the entire torque converter assembly is filled with transmission fluid which is shared with the rest of the transmission.
During operation, the engine spins the impeller, the vanes of which accelerate and pressurize the transmission fluid forcing the fluid toward the outer circumference of the torque converter. The pressurized fluid impinges upon the blades of the turbine, causing the turbine and the rest of the drive train to turn in the same direction as the engine. If resistance in the drive train is greater than the force exerted by the transmission fluid against the turbine blades, the turbine will remain stationary and the torque converter will "slip" allowing the vehicle to remain at a standstill.
As the turbine begins to rotate, the rate of rotation will be slower than that of the impeller, causing the transmission fluid to exit the turbine at an angle, and enter the stator. The blades of the stator further accelerate the transmission fluid and direct it back into the impeller at a higher pressure, where it is accelerated again by the impeller and returned to the turbine at an even higher pressure. The above-described flow of transmission fluid is referred to by those skilled in the pertinent art to which the invention pertains, as "vortex flow" and it is what creates the torque multiplication that gives vehicles equipped with automatic transmissions high low speed torque. The one way clutch in the stator prevents the stator from spinning in the wrong direction and destroying the vortex flow.
As the turbine speed approaches that of the impeller, centrifugal force causes the transmission fluid to be thrown from the turbine to the outer circumference of the torque converter preventing the fluid from being returned to the impeller. At the point where the turbine is spinning at roughly 90% of the impeller speed, the transmission fluid begins to impinge on the backs of the stator blades, unlocking the one way clutch and forcing the stator to turn in the same direction as the impeller and turbine. At this point there is no torque multiplication.
The one way clutch used in most torque converters is usually a sprag-type overriding clutch similar to that shown in FIG. 1. The illustrated clutch includes an outer race 10 and a cam stator 12 positioned within and coaxial with the outer race. The cam stator 12, includes a plurality of ramped portions 14 adjacent to a plurality of abutment walls 16. The outer race 10 and the cam stator 12 coact to define a plurality of pockets 18 located between successive abutment walls 16, each pocket having a roller 20 located therein. During operation, if a torque is applied to the outer race 10 in the direction indicated by the arrow labeled "Engaged", the rollers will move up the ramped portions 14 and become wedged between the cam stator 12 and the outer race 10 thereby preventing any relative rotation between the cam stator and the outer race. If the direction of the torque is reversed, the rollers 20 will be forced down the ramped portions 14, allowing the rollers to spin, causing relative rotation to occur between the outer race and the cam stator.
A problem occurs when attempting to employ the above-described torque converters in high performance situations such as drag racing, and is due to the fact that while the turbine is ramping up to the speed of the impeller, and the torque converter is operating to multiply torque to the driven wheels of the vehicle, tremendous heat is generated in the converter. In addition, exorbitantly large forces are exerted on the clutch components. The combination of temperature and force often cause the clutch to fail resulting in the destruction of the torque converter.
The same problem occurs at high stall speeds. Stall speed refers to the rate of engine rotation, e.g., engine RPM, that can be attained at full throttle with the vehicle's brakes locked and the transmission in gear, before the driven wheels turn. When performance modifications are made to a vehicle for, inter alia, drag racing, one of the objectives is to shift the engine's "torque curve" upward into a higher RPM range. In order to take advantage of this performance enhancement, the stall speed of the torque converter must also be increased accordingly. However, when converters employing one-way clutches of the type described above are used, the problem associated with temperature and excessive force is exacerbated, to the detriment of the clutch and thereby the converter. This is particularly true as the size of the torque converter decreases.
Based on the foregoing, it is the general object of the present invention to provide a torque converter that overcomes the problems and drawbacks of prior art torque converters.
It is a more specific object of the present invention to provide a stator/clutch assembly for a torque converter that can withstand high stall speeds.